The present invention relates to vehicle suspensions and in particular to the control of vehicle roll.
It is known for example from U.S. Pat. No. 4,050,704 to provide a vehicle suspension system in which roll, i.e. rotation of the vehicle about a longitudinal axis parallel to the normal direction of travel, is controlled by means of hydraulic cylinders or actuators.
The present invention provides a vehicle suspension system comprising a pair of suspension arms for supporting wheels on opposite sides of a vehicle, each suspension arm being pivotably attached to a vehicle body, and a pair of struts each arranged for connection between one of the suspension arms and the vehicle body to control rotation of the suspension arm about its respective pivot thereby to control vertical movement of the respective wheel relative to the vehicle body, wherein each strut comprises a cylinder and a piston which together define a working chamber, the, piston being movable relative to the cylinder to change the volume of the working chamber, the two working chambers are interconnected with each other, the struts are asymmetrically arranged and the cross sectional areas of the pistons and the distances between the struts and the respective pivots are arranged such that equal vertical movements of the wheels in the same direction produce equal but opposite changes in volume of the respective working chambers.
Preferably the pistons have equal effective working areas and the struts are connected to the suspension arms at equal distances from the respective pivots. However, provided the product of the working area of the piston and the distance between the pivot axes and the point of attachment of the strut to the suspension arm is the same on each side of the vehicle, equal wheel movements on each side of the vehicle will result in equal changes in volume of the respective working chambers and therefore equal volumes of fluid displacement on each side of the vehicle.
Preferably each piston has a front side and a back side and is connected to one of the body and the suspension arm by a connecting rod extending from the back side of the piston, wherein the working chambers are on the same side of the respective pistons. This enables identical struts to be used on both sides of the vehicle.
More preferably each piston divides its cylinder into first and second working chambers the two first working chambers having the same effective cross sectional area and being interconnected and the two second chambers having the same effective cross sectional area and being interconnected. This means that some fluid leakage around the piston between the working chambers is acceptable.
Conveniently one of the struts is connected to its suspension arm inboard of its pivot and the other is connected to its suspension arm outboard of its pivot, and the struts are preferably arranged on the same side of the suspension arms and are at least substantially parallel For example both the struts may be arranged to operate in use in a substantially vertical direction and may be arranged above the suspension arms. However it will be appreciated that the orientation of either of the struts can be chosen by choosing the direction at which the part of the suspension arm to which they are attached extends from the pivot. For example one or both of the struts can be arranged horizontally if it acts on a part of the suspension arm vertically above the axis of the respective pivot.
At least one of the pairs of interconnected clambers may be connected to a source of fluid pressure and control means provided to control the flow of fluid to and from said pair of chambers to control actively roll of the vehicle.